CHAPTER V. 



THE LOWER SILURIAN BRACHIOPOD A 



OF MINNESOTA. 



BY N. H. WINCHELL AJN'D CHARLES SCHOCHEKT 



PRESERVATION AND DISTRIBUTION. 



In the upper third of the Trenton limestone in the vicinity of Minneapolis, 

 brachiopods are numerous, but usually not very well preserved. This is due mainly 

 to the dolomitic nature of the rock, and the frequent small crystals of calcite and 

 pyrite lining the cavities of the casts. In some of the shaly layers, slabs are found, 

 containing very fine specimens of Rhynchotrema inwquivalvis, Orthis deflecta, 0. sub- 

 cequata var. conradi, Scenidium anthonensis and Rafinesquina minnesotensis. With a 

 few exceptions, all the species of this division also occur in the Trenton shales above. 



In the Trenton shales, a great abundance and variety of forms of well preserved 

 fossils can be gathered readily in the immediate vicinity of St. Paul and Minne- 

 apolis. The greatest thickness of the shales is about seventy feet, but towards the 

 southern part of the state they rapidly diminish, so that at Preston, which is near 

 the state line in Fillmore county, they are not more than fifteen feet thick. The 

 predominating fossils of this horizon are bryozoans, and next in abundance are the 

 brachiopods. The latter are beautifully preserved, either as entire specimens or 

 separated valves, so that the external and internal structure of nearly all the species 

 can be understood satisfactorily. Much can also be accomplished in the discovery 

 of young specimens 1 mm. in size up to maturity. These small specimens cannot 

 be picked up on the hill sides, nor on the quarry dumps, but usually where adult 

 examples of a species are abundant, there, also, will be found all individuals from 

 the youngest to the mature shells. Collectors discovering such localities should not 

 fail to carry away a small sample of the shale to be washed carefully in a pan until 

 the water is colored no longer by the residuum. After drying, what remains should 

 be sifted into various sizes to facilitate examination with the hand lens. If the 

 sample proves to contain young specimens, it will be only a matter of washing and 



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